CPO code needs more tests

[gassmoeller]

As #5332 shows the CPO code is currently insufficiently tested. This is because tests were only possible when all the major components were in place. However, they are merged now, so we should add the necessary tests. @MFraters can you take a look?

Related to #3885.

[MFraters]

Yes, test are in the planning, and @srivatsvyas has already done some of the testing because the drexpp branch is based on this version instead of the paper version.

I would like #5116 to be finished first so that the results can be completely compared in the integration tests. (There are already a lot of unit tests)